The present invention relates generally to integrated circuits exposed to light and more generally to protecting such integrated circuits from damaging photo-induced currents.
Integrated Circuits (ICs) are widely used in industry. Among the applications are as analog elements that use analog charge storage elements, and as digital elements such as dynamic random access memories (DRAMs) that use analog charges stored on capacitors to represent binary digits. Because the storage of charge represents valuable data, it is important in all such applications to precisely preserve the value of the charge that has been stored so the data is stored in an uncorrupted state. The data will become corrupted if the charge is changed in any way. Therefore, the IC must be protected from anything that might corrupt the data, such as leakage currents, or in this case, currents induced by light falling on the IC.
Silicon, the main material used in constructing ICs, is a good visible light photocurrent producer. When light around the visible and infrared spectra, about 4,000 xc3x85 to 8,000 xc3x85, falls on the IC, the energy of the light can produce free electrons, or a xe2x80x9cphoto-induced currentxe2x80x9d, in the silicon.
In some applications, ICs are used to form an image and they are intentionally exposed to light. Such ICs take advantage of photo-induced currents and are designed to convert light into electrons of measurable current in order to convert the visual image into electronic data.
While beneficial in some parts of the chip, the photo-induced currents can be damaging in others.
There are many specific applications where light threatens the integrity of stored data. Display devices are a prime example because the ICs are exposed to significant amounts of light. In these applications, the electrons of the photo-induced current can either bleed off the charge that is stored on a display device""s storage element or add to the existing charge, in both cases corrupting the stored data, which results in deterioration in the quality of the resulting display device image.
The display devices have a metallic array of electrodes on the top surface with transistors and charge storage elements making up the IC underneath. On the top level, reflective aluminum electrodes form the display by reflecting light. In order to see the image on the display, the circuit must be illuminated with large amounts of light. The gaps between the electrodes are minimized to try to reduce the amount of light that reaches the silicon layer, but significant amounts still reach the silicon potentially leading to corrupted data. Projection applications in particular, where the intensity is extremely high, have been affected by this problem. Light leaking through gaps in the electrodes has in some cases changed the stored charge 50% from its original value. The resulting display errors can only be compensated by reducing either the contrast or the brightness of the display, which limits the range of those settings to often-undesirable levels.
Thus, a system for protecting ICs which may be detrimentally affected by photo-induced currents has been long sought but has eluded those skilled in the art.
The present invention provides an apparatus and a method for protecting charge storage elements from photo-induced currents in silicon integrated circuits. In order to protect against photo-induced currents that are generated outside the storage node circuits themselves, an n-well guard ring is placed as closely as possible to the transistors and other elements in the storage node circuits. This structure minimizes the exposed silicon area in which light can produce current in areas next to the storage node circuits. The n-well guard ring captures the photo-induced currents that are generated outside the storage node circuits. In order to protect against the photo-induced currents that are generated inside the storage node circuits, an aluminum interconnect layer is placed on top of the storage node circuit, separated by an insulating layer of silicon dioxide. This creates a shield against the light and protects the storage node circuit by reflecting light away.
The above and additional advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description when taken in conjunction with the accompanying drawings.